I always thought it was a way to derive profit from not only people's accommodation, but also whatever else they want to spend their time doing on their holidays.
That was my thought. Same as ads. If the currency is the attention of users, why not double-dip profits!
It's the same reason when I book a plane flight, someone's always trying to sell me the rental car while I'm there
It's both but there's no doubt they were feeling squeezed in numerous locales for having people skirting or outright violating regulations on hotels and vacation properties.
Partly that, but it’s also a way to differentiate a largely generic product that mostly acts as a middleman. There are already a few Airbnb clones, and there are really not very high barriers to switch platforms. In the long term Airbnb needs to add value beyond the basic marketplace.
Probably part of it, but I'm pretty sure they did it because they essentially saturated the current market and in order to grow (and/or pump up their valuation) they had to reach outside their core.